Traditionally, shot for shotguns has been composed of lead by virtue of its high density and low melting point characteristics. In recent years, however, lead has fallen into disfavor owing to its toxicity. On the other hand, there are no satisfactory substitute metals possessing the same density characteristics, and those metals that are somewhat close to lead in density are not satisfactory substitutes as a result of other drawbacks, such as, high cost, radioactivity, high melting point or other properties. Accordingly, numerous attempts have been made to formulate a mixture of metals which would serve as satisfactory substitutes for lead and especially in the manufacture of shot, pellets, bullets and the like.
Among other approaches which have been proposed, U.S. Pat. No. 4,428,295 to V. Urs is directed to a high density shot made up of an unsintered, cold-compacted mixture of at least two metal powders, one of the powders being more dense than lead and a second one being about the density of lead and flowable under compaction to serve as a matrix that surrounds the denser unmelted powder. The patent to Urs in particular is representative of approaches which have been taken to achieve at least the density of lead by combining lead with the powder of a metal that is more dense than lead. Urs avoids sintering in combining or compacting the metals together, as a result of which the end product has cold welding lines with microscopic voids or air pockets along those cold welding lines which weaken the product. The term "sintering" as employed in the metallurgical industry is the treating of compacted metal powders by heating to an elevated temperature sufficient to cause diffusion without melting of any of the metals present. One difficulty in sintering a single low melting point metal is that temperature and time are hard to control to the required tolerances and, for example, heating even slightly above the melting point temperature can result in melting of the metal into a puddle. On the other hand, sintering of the low-melting-point metal is desirable from the standpoint of achieving higher values of density and strength of the resultant article, because sintering is more effective than compaction alone in causing the matrix to become continuous and avoid weld lines in the article.
U.S. Pat. No. 4,949,644 to J. E. Brown utilizes bismuth or a bismuth alloy in the formation of high density shot. However, achieving the density of lead in this manner is exceedingly difficult since bismuth is significantly less dense than lead, and to alloy bismuth with any of the few metals that are more dense than lead poses immense problems of toxicity, economy or high temperature processing.